1. Field of the Invention
This invention relates to integrated circuit manufacturing technology and, more specifically, to processes for growing insulative layers by consumption of an underlying layer.
2. State of the Art
In the manufacturing of integrated circuits, the growth of high-quality insulative layers for device isolation, transistor gate dielectrics and capacitor dielectrics is of fundamental importance. As device dimensions are scaled down, thinner dielectric layers are required to maintain capacitor and transistor performance at acceptable levels. When silicon dioxide layers, which have been grown by conventional wet or dry thermal oxidation processes, are thinned, device reliability typically suffers. It is generally believed that the quality of silicon dioxide layers is inversely proportional to the number of interface states (i.e., dangling silicon bonds) present per unit volume. In other words, quality and long-term reliability of a silicon dioxide layer will increase as the number of dangling silicon bonds is reduced.
Oxidation of silicon in an ambient-containing nitrous oxide (N2O) has been reported as a means for improving the quality of silicon dioxide dielectric layers. A lower number of interface states, greater reliability, improved hot carrier immunity and improved diffusion barrier characteristics have been reported for silicon dioxide layers grown in this manner as compared to those grown via conventional dry or wet oxidation processes. The increased reliability of such dielectric layers may be due in part to the incorporation of nitrogen atoms into the silicon dioxide matrix. The bonds between silicon atoms and nitrogen atoms are considerably stronger than those between silicon atoms and oxygen atoms. Thus, the presence of nitrogen atoms in the matrix apparently fortifies the dielectric layer against voltage-induced degradation. However, the process of oxidizing silicon in nitrous oxide tends to be self-limiting because the silicon dioxide layer which forms has a high nitrogen content. In fact, the layer may be better identified as silicon oxynitride. Once a thin layer of silicon oxynitride has formed on the surface, the diffusion of oxidant species to the underlying silicon is greatly hampered. Thus, only very thin dielectric layers are readily grown using this process.
Although the addition of small amounts of a strong oxidizer such as SF6 or NF3 to a nitrous oxide ambiance has been demonstrated to increase the oxidation rate by as much as one order of magnitude, the inclusion of fluorine in a silicon dioxide layer has been shown to have an adverse impact on device reliability.
What is needed is an improved process for oxidizing silicon which utilizes nitrous oxide as the primary oxidant species, and which does not rely on fluorine-containing additives to overcome the self-limiting nature of the reaction. Such a process might produce silicon dioxide layers having both a low number of interface states and a sufficient thickness for use as transistor gate dielectrics.
An improved process for growing high-quality silicon dioxide films or layers on a silicon surface in a nitrous oxide-containing ambiance is disclosed. The silicon surface is subjected to a gaseous mixture of nitrous oxide (N2O) and ozone (O3). The presence of O3 in the oxidizing ambiance greatly enhances the oxidation rate compared to an ambiance in which N2O is the only oxidizing agent. It is hypothesized that the presence of O3 promotes growth of a dielectric layer that is predominantly silicon dioxide, although small numbers of nitrogen atoms are incorporated in the silicon dioxide matrix. Thus, a silicon oxynitride layer (i.e., a silicon dioxide layer having a high nitrogen content) having a high degree of impermeability to the oxidant species is prevented from forming. The presence of O3 in the oxidizing ambiance does not impair oxide reliability, as is the case when silicon is oxidized with N2O in the presence of a strong, fluorine-containing oxidizing agent such as NF3 or SF6.